Opioids and opiates are potent analgesics widely used in clinical practice. Opiates refer to alkaloids extracted from poppy pods (Opium Poppy; Papaver Somniferum) and their semi-synthetic counterparts which bind to the opioid receptors. Basically to be called an opiate one has to either be a natural opioid receptor agonist or start the refining process with one of the natural alkaloid molecules. Once chemically altered, such as the process of converting morphine into heroin, the drug is then labeled as a semi-synthetic opiate or semi-synthetic opioid—the terms can be used interchangeably. Semi-synthetic opiates (or semi-synthetic opioids) include heroin (diamorphine), oxycodone, hydrocodone, dihydrocodiene, hydromorphone, oxymorphone, buprenorphine and etorphine. In contrast, opioid is a blanket term used for any drug which binds to the opioid receptors. Opioids include all of the opiates as well as any synthesized drug that bind to opioid receptors. Synthetic opioids include methadone, pethidine, fentanyl, alfentanil, sufentanil, remifentanil, carfentanyl, tramadol, tapentadol and loperamide.
Opioid and opiates drugs are classified typically by their binding selectivity in respect of the cellular and differentiated tissue receptors to which specific the drug binds as a ligand. There are 3 well-defined or “classical” types of opioid receptor: mu (μ), delta (δ), and kappa (κ). More recently, cDNA encoding an “orphan” receptor named ORL1 (opioid receptor-like) was identified which has a high degree of homology to the “classical” opioid receptors. All the opioid receptors are G-protein coupled receptors and possess the same general structure: an extracellular N-terminal region, seven transmembrane domains and an intracellular C-terminal tail structure. Pharmacological evidence supporting for subtypes of each receptor and other types of novel, less well-characterised opioid receptors have also been postulated. The well-known opioid analgesics bind to and activate selectively the opioid mu receptors; that is, they act as agonists at mu opioid receptors. The sigma receptor, however, is not regarded as an opioid receptor.
Opioid analgesics are recommended for the management of moderate to severe pain including that which occurs following surgery and trauma and in many patients with cancer. Apart from pain relief, opioid analgesics also produce a range of common well-known side effects (e.g., sedation, emesis, constipation, respiratori depression, dependence).
In addition to the afore-mentioned side-effects, it has been appreciated more recently that opioid analgesics may also activate a pro-nociceptive mechanism resulting in the phenomenon of opioid-induced hyperalgesia (OIH) [also called opioid-induced abnormal pain sensitivity]. OIH is a recognized complication of opioid therapy characterized by enhanced pain sensitivity. Somewhat paradoxically, opioid therapy aiming at alleviating pain may render patients more sensitive to pain and potentially may aggravate their preexisting pain. In fact, OIH should be considered in the differential when opioid therapy fails. Hence, any apparent decrease in opioid analgesic effectiveness may be due at least in part to the presence of OIH rather than reflecting a worsening of the disease state and/or the development of pharmacological tolerance.
As disclosed in the art (Sandford, M. et al.; Pain Physician 2009; 12:679-684) the existence of OIH is proved by basic science evidence (Mao, J.; Pain 2002; 100:213-217) and by clinical evidence (Guignard, B. et al.; Anesthesiology 2000; 93:409-417 and Angst, M. S. et al.; Anesthesiology 2006; 104:570-587). Additionally there are neurobiological mechanisms discussed for OIH involving the central glutaminergic system, the spinal dynorphins or the descending facilitation.
OIH is evidenced by individuals taking opioids, which can develop an increasing sensitivity to noxious stimuli (hyperalgesia), even evolving a painful response to previously non-noxious stimuli (allodynia). Increased pain in OIH may result from one or more of the following: pain in the absence of a noxious stimulus (spontaneous pain), increased duration of pain in response to brief stimulation (ongoing pain or hyperpathia), reduced pain threshold (allodynia), increased responsiveness to suprathreshold stimulation (hyperalgesia), spread of pain and hyperalgesia to uninjured tissue (referred pain and secondary hyperalgesia), and abnormal sensations (e.g., dysesthesia, paresthesia).
OIH is a phenomenon often associated with the long term use of opioids, but some studies have demonstrated that this effect can also occur after only a single dose of opioids (E. Celerier et al., J. Neurosci. 21, 4074-4080 (2001)). Thus, OIH occurs following both acute and chronic opioid administration. In this way, OIH is a less recognized side effect of chronic opioid therapy. However, it is becoming more prevalent as the number of patients receiving opioids for chronic pain increases (Trescot, A. M. et al.; Pain Physician 2008; 11:S12-S16).
Increases in pain intensity can occur upon discontinuation of opioid therapy but such an abnormal increased pain sensitivity including hyperalgesia or allodynia can occur also in the absence of overt opioid withdrawal in subjects that have been administered opioid drugs.
The cellular mechanisms underpinning OIH have been proposed to be in common with those of neuropathic pain and analgesic tolerance involving augmented glutamatergic signaling and persistent activation of the N-methyl-D-aspartate (NMDA)-nitric oxide synthase (NOS)-nitric oxide (NO) signaling cascade.
Another mechanism proposed to underpin opioid-induced excitatory signaling involves stimulation of adenylate cyclase formation via G5-coupled opioid receptors that opposes inhibition of adenylate cyclise formation via Gi/o-coupled opioid receptors to attenuate levels of pain relief (Smith, M. T.; Acute Pain 2008; 10:199-200).
It is known that the combination of opioid analgesics with agents that block excitatory opioid signaling pathways can improve pain relief. Some strategies include combining opioid analgesics with NMDA-receptor antagonists, such as low dose ketamine, and more recently, clinical trials have investigated combinations of ultra-low dose naltrexone (non-selective opioid antagonist) and opioid agonists such as morphine and oxycodone to selective block signaling via G5-coupled opioid receptors (Smith, M. T.; Acute Pain; 2008; 10; 199-200) that are useful in the prevention and/or treatment of opioid-induced hyperalgesia.
Sigma receptors are non-opioid receptors of great interest in pharmacology. The sigma binding sites have preferential affinity for the dextrorotatory isomers of certain opiate benzomorphans, such as (+)SKF 10047, (+)cyclazocine, and (+)pentazocine and also for some narcoleptics such as haloperidol. The sigma receptor has at least two subtypes, which may be discriminated by stereoselective isomers of these pharmacoactive drugs. SKF 10047 has nanomolar affinity for the sigma 1 (σ-1) site, and has micromolar affinity for the sigma 2 (σ-2) site. Haloperidol has similar affinities for both subtypes.
It has been reported that some sigma receptor ligands (i.e., haloperidol) in combination with opioids are capable of modulating the analgesic effect of opioids (both kappa and mu opiods) in models of acute thermal nociceptive tests (i.e., radiant heat tail-flick test) in mice (Mei J and Pasternak G W, Sigma 1 receptor modulation of opioid analgesia in the mouse, J Pharmacol Exp Ther. 2002, 300(3):1070-1074) and rats (Chien C C and Pasternak G W, Sigma antagonists potentiate opioid analgesia in rats, Neurosci Lett. 1995, 190(2):137-139). Recently it has been shown that some sigma-1 receptor antagonists potentiate opioid analgesia in models of acute thermal nociceptive pain and that this potentiation of analgesia is not accompanied by potentiation of opioid side effects (i.e., dependence) (WO 2009/130310). However, no information is available regarding inhibition of OIH by sigma-1 receptor ligands.
The treatment of OIH can be time-consuming and, at times, impractical. Weaning patients from high dose opioids usually requires time and patience. While reducing the opioid dose, patients may experience transient increases in pain or exacerbation of pain and the hyperalgesic effect may not be mitigated until a certain critical dose of opioid is reached.
Breaking the cycle of OIH is an attractive course of action for the interventional pain specialist. Thus, there is still a need for substances that could be used as an adjuvant to opioid therapy for the prevention and/or treatment of the associated OIH.